A helical antenna comprises one or more elongated conductive elements wound in the form of a screw thread to form a helix. The geometrical helical configuration includes electrically conducting elements of length L arranged at a pitch angle P about a cylinder of diameter D. The pitch angle is defined as an angle formed by a line tangent to the helical conductor and a plane perpendicular to a helical axis. Antenna operating characteristics are determined by the helix geometrical attributes, the number and interconnections between the conductive elements and the feed arrangement. When operating in an end fire or forward radiating axial mode the radiation pattern comprises a single major pattern lobe. The pitch angle determines the position of maximum intensity within the lobe. Low pitch angle helical antennas tend to have the maximum intensity region along the axis; for higher pitch angles the maximum intensity region is off-axis.
Quadrifilar helical antennas (QHA) are used for communication and navigation receivers operating in the UHF, L and S frequency bands. A resonant QHA with limited bandwidth is also used for receiving GPS signals. The QHA has a relatively small size, excellent circular polarization coverage and a low axial ratio over most of the upper hemisphere field of view. Since the QHA is a resonant antenna, its dimensions are typically selected to provide optimal performance for a narrow frequency band. C. C. Kilgus first described the QHA in “Resonant Quadrifilar Helix,” IEEE Transactions on Antennas and Propagation, Vol. AP-17, May 1969, pp. 349–351.
One prior art quadrifilar helical antenna comprises four equal length filars mounted on a helix having a diameter of about 30 mm for operation at about 1575 MHz. Given these geometrical features, the antenna presents a driving point impedance of about 50 ohms, which is suitable for matching to a common 50 ohm characteristic impedance coaxial cable. The four filars of the QHA are fed in phase quadrature, i.e., a 90 degrees phase relationship between adjacent filars. There are at least two known prior art techniques for quadrature feeding of the four equal-length QHA filars. One such quadrature matching structure employs a lumped or distributed branch line hybrid coupler (BLHC) and a terminating load, together with two lumped or distributed baluns. Another technique that offers a somewhat broader bandwidth, uses three branch line hybrid couplers (a first input BLHC receiving the input signal and providing an output signal to two parallel BLHC'S) each operative with a terminating load. A quarter wave phase shifter provides a 90 degrees phase shift between the first BLHC and one of the parallel-connected BLHC'S.
It is known that such quadrature matching techniques, such as hybrid couplers and baluns, disadvantageously increase the size of the printed circuit board on which the antenna is mounted. The couplers and baluns also increase the antenna cost, and each additional component operative with the antenna imposes losses and bandwidth limitations.
It is further known in the prior art to construct a QHA comprising a first and a second filar having unequal lengths, i.e., a long and a short filar. Each filar further comprising a first and a second conductive element. The first filar comprises a coaxial cable having a center conductor connected to an antenna feed terminal at a bottom end of the QHA and a shield connected to an antenna ground terminal. The second filar comprises a conductive wire. At a top end of the QHA, the coaxial cable shield is connected to the first element of the second filar and the center conductor is connected to the second element of the second filar. At the bottom end, the coaxial cable center conductor (comprising the first filar) is connected to the shield and the first and second elements of the second filar are connected together.
Typically, the QHA is a self-sufficient radiating structure operated without a ground plane or counterpoise. However, when the QHA is installed in close proximity to a radio transceiver handset, the handset structure can induce electromagnetic wave reflections that influence the QHA's radiation pattern and impedance, much like a ground plane. For example, if the QHA emits a right-hand circularly polarized signal, upon reflection from a conducting surface, the signal is transformed to a left-hand circularly polarized signal. Obviously, such effects negatively influence the antenna's performance, and can be particularly troublesome if the communications system employs dual signal polarizations.